The present invention relates to an FM signal demodulation system and, in particular, relates to such a system which improves the noise characteristics of the demodulated signal in a frequency modulation (FM) signal.
A prior FM signal demodulation circuit is a frequency demodulation circuit having an inductance (L) and a capacitance (C), or a delay line. In such case, the relationship of the signal-to-noise ratio (S/N) of the demodulated signal for the ratio (C/N) in which C is the carrier signal power and N is the thermal noise power, of an input signal is: EQU S/N=C/N(FI)
where FI is a constant. Therefore in general, the value S/N is proportional to C/N. However, when the value C/N is less than a predetermined threshold level, the above equation is not satisfied, and the value S/N deteriorates considerably. That threshold level is usually approximately 10 dB. In order to improve the value C/N, a bandpass filter is utilized at the input of the demodulator in order to restrict the bandwidth of the input signal and the thermal noise.
In a television signal transmission system through a satellite relay system, an FM (Frequency Modulation) system has been used. The reception signal level in a satellite communication system is usually around the threshold level, due to the restriction of the transmission power of a satellite, the attenuation and/or stability of a transmission path, and/or the apparatus of an earth station. Accordingly, when the reception signal level is decreased a little for some reason, that signal level becomes lower than the threshold level, and then, the picture quality of a television signal is considerably deteriorated by spike noise which is peculiar to a television transmission, and/or it becomes even impossible to reproduce a picture signal.
Accordingly, it would be desirable to decrease the influence of the spike noise, and/or the decrease of the reception level of a television signal, in order to improve picture quality. In particular, in the case of a direct satellite broadcasting system in which a satellite transmitter sends a picture signal directly to consumer television reception sets, the improvement of noise characteristics is very important.